Rotary fluid equipment, such as centrifugal pumps, usually include a rotary operative member such as an impeller which is driven by a prime mover such as an electrical motor. A rotary shaft connects the prime mover to the operative member. Typically the operative member is contained within its own housing, having an inlet and an outlet for the fluid being moved, with the shaft passing through the back of the housing. The shaft may be supported by bearings and usually there will be seals engaging the shaft to prevent fluid which enters the housing from reaching the prime mover. The seals, shaft and seal housing define a seal cavity which opens towards the housing containing the operative member.
During operation of the rotary fluid equipment particulate contaminant material carried into the equipment by the moving fluid, or eroded from the adjacent surfaces of the housing or the impeller, can enter the seal cavity and damage the shaft and/or the seals unless some type of protector is arranged at the entrance to the seal cavity.
There have of course been attempts in the past to protect the seal cavity. Most such attempts have involved the attachment of a plate or other member to the shaft at the entrance to the seal cavity to aid in eliminating air (U.S. Pat. No. 3,070,028) or to prevent fluid from entering the seal cavity when the equipment comes to a halt (U.K. Patent 212,879). Other attempts involve labyrinth type seals which utilize elements on the shaft and the shaft housing which are interdigitated to increase the difficulty for contaminants to enter the seal cavity (U.S. Pat. Nos. 2,834,618; 2,874,982; 4,185,838; and 4,335,886).
U.S. Pat. No. 4,872,690, assigned to the assignee of the present application, describes a device which is successful in reducing contaminant concentration in the seal cavity. It does, however, require intricate machining to install and it requires a high velocity source, such as a rotating impeller, very close thereto for successful operation. Operation of the device results in a marked reduction in the seal cavity pressure. The reduced pressure draws a very high flow of contaminated fluid from behind the impeller into the device. In drawing a high flow into the device, high wear occurs on the shaft and components of the device. Additionally, the reduced seal cavity pressure and the very high exchange rate of fluid into the device and the seal cavity prohibits its use in applications wherein the pumped fluid is at or even near its vapour point. The high fluid exchange and pressure reduction can result in flashing of the product to vapour at the seal faces, thus destroying the seal. The requirement for a high velocity source prohibits use of the '690 invention in certain low speed pump types, including progressive cavity pumps, screw pumps and gear pumps.